In general, polyimide is applied to various fields including insulating substrates for forming circuits and devices due to excellent mechanical and heat properties. However, since a charge transfer complex is formed between aromatic rings during polymerization, the polyimide is colored with brown or yellow color, and thus, a transmittance in a visible light region is low, which has difficulty in being applied to display materials.
As a method of preparing the polyimide to be colorless and transparent, a method of suppressing formation of the charge transfer complex in a molecule by using alicyclic diamine or an aliphatic diamine as a diamine component has been known. Japanese Patent Laid-Open Publication No. 2002-161136 (Patent Document 1) discloses polyimide obtained by imidizing a polyimide precursor formed with an aromatic acid dianhydride such as pyromellitic dianhydride, etc., and trans-1,4-diaminocyclohexane. The polyimide exhibits high transparency, but has a problem in that mechanical properties are deteriorated. That is, it has been known to use various functional monomers as a method for converting the yellow color of polyimide to be colorless and transparent, but there is still a problem in that inherent and excellent mechanical properties are deteriorated. In addition, the transmittance of the transparent polyimide is high at a long wavelength (that is, higher than 450 nm). On the contrary, the transmittance of the transparent polyimide is rapidly reduced at a short wavelength (that is, lower than 450 nm).
Meanwhile, a scope at which display materials are applied has been diversified according to development of various electronic devices, and research and development for replacing a cover glass for display with a polymer material are actively progressing. Therefore, it is required to develop technologies for improving optical and surface properties simultaneously while implementing inherent and excellent physical properties of polyimide in various display material fields including materials for replacing the cover glass.